(Updated) Children who suffer concussions may experience lingering problems with memory and attention, and may need help in school, according to a new study in the Archives of Pediatrics & Adolescent Medicine.

The study found that most children who bumped their heads didn’t show serious symptoms in the weeks after their injury, but a small group did go on to suffer increasing cognitive and physical symptoms, including headache, fatigue, forgetfulness and inattentiveness, in the two weeks following their concussion. Kids who suffered concussion were also more likely to show cognitive symptoms than children with orthopedic injuries; over time, those differences started to shrink, but the concussion group continued to show more cognitive symptoms even 12 months after their head injury.

Children who lost consciousness were more likely than kids who weren’t knocked out to show more symptoms afterward: 20% continued to have forgetfulness and fatigue after their accident.

“Our study confirms what a lot of us thought, which is that many, many kids who have concussions in fact do very well,” says lead author Keith Yeates, director of the Center for Biobehavioral Health at the Research Institute at Nationwide Children’s Hospital in Columbus, Ohio. “That’s the good news. But the not-so-good news is that there does seem to be a small proportion of kids who don’t recover fully, and go on to have more persistent problems.”

The study involved 186 children aged 8 to 15 who experienced concussion — most commonly in a fall or while playing sports — or other brain injury from a car accident or other causes. The children’s parents were asked to fill out a standard questionnaire to assess their child’s cognitive and physical status before the concussion, and again two weeks, three months and a year later. These responses were compared to those from parents of 99 similar children who came into the emergency room with orthopedic injuries.

About 500,000 children under 15 experience a concussion or mild traumatic brain injury serious enough to require hospitalization each year. In Yeates’ study, most of the injuries came from sports, with 20% resulting from car accidents, falls or other types of trauma.

“We aren’t yet at the point of being able to identify, at the time of injury, who is going to have problems and who isn’t,” says Yeates. “Our study is the first step to say that there are features that seem to stand out and maybe increase the risk of having these problems, but we still can’t readily tell families which kids are at particularly high risk.”

Kids with lasting symptoms may need short-term assistance in school, such as having tutors or being given extra time to take tests or complete assignments.

The lesson, according to Dr. Frederick Rivara of the University of Washington and Seattle Children’s Hospital, is that emergency room physicians need to be more vigilant about recognizing and monitoring children who come in with concussions. “The overall message emerging from this research is that the group of injuries classified as ‘mild TBI,’ including sports-related concussions, should not necessarily be treated as minor injuries, which quickly resolve,” Rivara wrote in an editorial accompanying the new study.

He notes that the latest data on concussions emerging from the military finds that concussion symptoms can hide behind normal brain scans. In a recent study, 29% of soldiers with blast-related TBI (traumatic brain injury) had normal brain scans. Only when doctors used more sophisticated imaging techniques did they find evidence of abnormalities related to the concussion.

In recent years, the NFL has also started taking concussions more seriously, following a spate of brain injuries and related deaths. In December, the league appointed a concussion referee to assess players for risk of brain trauma on the field.

Along with Yeates and other concussion experts, Rivara believes more research into finding the markers for concussion symptoms is necessary. Without objective ways to determine how badly a brain is affected by a blow, it will continue to be difficult to advise parents and to treat children who have had concussions.